The object of the invention is a method for determining channel information in a cellular system, where a TDMA (Time Division Multiple Access) transmission protocol is used on the traffic channel allocated to the user traffic connection during the user traffic connection between a mobile station and a base station of the current cell, and a mobile station realising the method. The invention is advantageously applied in a system which utilises a number of time slots of the TDMA frame to transmit information, such as in a system utilising the so called HSCSD protocol (High Speed Circuit Switched Data). Most advantageously the invention can be used in WLL (Wireless Local Loop) terminals.
Information about a base station in a neighbour cell is transmitted from the base station to the mobile station, i.a. for synchronising to the neighbour cell base station and for performing level measurements. In order to understand the invention a prior art neighbour cell monitoring in a cellular network is described in more detail below using a digital GSM system (Global System for Mobile communications) as an example.
In the GSM system separate frequency bands are allocated to transmission and reception, and on each frequency the data is transmitted as bursts in the slots of a TDMA frame. The TDMA frames contains eight time slots, of which one or more are allocated to the connection between the mobile station and the base station.
A mobile station operating in a cellular network needs information about the base stations of the active cell and of the base stations in the other cells around the mobile station so that it is able to perform a flexible handover when required. FIG. 1 shows a cell (Serving cell), which serves a mobile station of the system, and the six other cells (Cell 1 to Cell 6) which are located around it. The mobile station measures the signal levels (RXLEV) which it receives from the base stations of these cells, and reports the measurement data to the serving base station. In the GSM system each base station has a certain transmission frequency, a so called broadcasting frequency, at which the base station continuously transmits with a constant power. The mobile station measures the power received from the base stations at the broadcasting frequency of respective base station. In the following said signal level measurement (RXLEV) of the neighbour base stations is called “neighbour cell base station level measurement”.
The mobile station must also receive the Base Station Identity Code (BSIC) of each base station so that the mobile station knows which base station's signal level it measures at each frequency. Each base station transmits regularly the identity code. One time slot of the TDMA frames transmitted at the broadcasting frequency, the time slot “0”, is allocated to channels which simultaneously transmit information to a plurality of mobile stations, i.a. for synchronising to the base station. Such channels in the GSM system are i.a. the following: the Frequency Correction CHannel (FCCH), the Synchronisation CHannel (SCH), the Broadcast Control CHannel (BCCH), and the Common Control CHannel (CCCH). Fifty-one TDMA frames form a so called 51-multiframe (Multi Frame). Regarding the above mentioned channels it is specified in which TDMA frame of the multiframe they are located. A mobile station looks for and decodes the channels located in said TDMA frame of the broadcasting frequency among the neighbour base station transmissions. Said base station identity code BSIC is transmitted on the synchronisation channel SCH.
The above mentioned function of a mobile station for receiving information transmitted by a neighbour base station is in the following called “reception of neighbour base station information”. The level measurement (RXLEV) of a neighbour cell base station and reception of neighbour cell base station information (BSIC) is for short called “neighbour cell monitoring”.
FIG. 2 shows the TDMA frame structure of the downlink in the GSM system and the moments when the neighbour cell monitoring is performed. The transmission and reception is presented in the figure as mobile station functions, whereby TX means data transmission on the uplink and RX means data transmission on the downlink. The TDMA frames 21, 23 and 24 contain eight time slots, of which the time slot “0” is used for data reception RX, and the data transmission TX to the base station occurs during the time slot “3”. The time slot “0” in the uplink TDMA frame is located at the time slot “3” in the downlink TDMA frame, because there is a timing difference of three time slots between the downlink and uplink TDMA frames. Thus there are two unused time slots between the reception RX and transmission TX, and during these two time slots the frequency synthesiser switches from the reception frequency to the transmission frequency. Then four unused time slots are left at the end of normal TDMA frames, during which the neighbour base station level measurements are made, period 26.
A mobile station receives neighbour base station information during empty frames (so called Idle frames), every 26th TDMA frame of the frames transmitted by a base station is such an empty frame. No speech/data is transmitted in neither direction in the cell in question during an idle frame. The idle frames and the above mentioned 51-multiframes are arranged in sequences with different lengths, 26 and 51 TDMA frames, so that the SCH channel burst can be received in at least every eleventh idle frame, as is shown in the example of FIG. 2 during the shown idle frame 22, period 25. Either before or after an idle frame the transmitted frames also contain time slots which are not used by the user traffic connection, which unused time slots together with the idle frame in this case form a period of 12 time slots, during which period the neighbour base station information can be received. The neighbour base station information can also be received during the time slots of such normal TDMA frames in which the mobile station itself does not receive or transmit information of the user traffic connection.
In known solutions the time required for neighbour cell monitoring may become a problem. Each surrounding base station broadcasts on a different frequency, and therefore the frequency synthesiser of the mobile station must be able to switch sufficiently rapidly to the examined frequency so that the monitoring can be performed. When the monitoring has been performed the frequency synthesiser must rapidly return to a frequency where it can either receive or transmit information on the user traffic connection.
Problems may occur in the new broadband GSM2+, HSCSD (High Speed Circuit Switched Data) and GPRS (General Packet Radio Service) services, because in them the connection's traffic channel utilises more time slots of the TDMA frame than in the previous basic systems. FIG. 3 presents as an example a frame structure which is used in a mobile station according to the HSCSD class 12 MS. In said class it is possible to use a total of five time slots out of the eight in a frame, so that the majority of the time slots are allocated to reception. In the example of FIG. 3, of the eight time slots belonging to the frame three time slots are used for reception RX and two time slots for transmission TX. In FIG. 3 the TDMA frame RX of the downlink and the TDMA frame TX of the uplink are presented as separate frames.
The HSCSD classes include full duplex systems in which a mobile station can simultaneously both transmit and receive information. However, in the case of FIG. 3, HSCSD class 12 MS, the mobile station is in a half duplex operating state. Of all half duplex HSCSD classes this class presents the highest requirements on the frequency synthesiser. In the case of FIG. 3 the neighbour base station level measurement 32 made within the TDMA frame at the interface between the transmit time slots 3 and 4 requires a frequency hop to the examined frequency 31 before the measurement. After the measurement 32 a new hop is made to the reception frequency 33 of the traffic channel.
In addition to the neighbour base station level measurement made within the TDMA frame used by the traffic channels there are also made level measurements during the idle frame and the idle time slots adjacent to it. The corresponding period is it the following called the “Idle period”. In the example case of FIG. 4 the length of said period 41 is 10 time slots. FIG. 4 shows the TDMA frames transmitted on the broadcasting frequency of a neighbour base station, and the time slots 42, 43 and 44 allocated to the FCCH, SCH and CCCH channels in these TDMA frames. As is observed in FIG. 4 the time slot S of the synchronisation channel is in this case located at the very beginning of the available reception period, whereby the frequency synthesiser has not yet had time to settle on the broadcasting frequency of the neighbour base station. When the settling time with a length of about one slot of the frequency synthesiser is taken into account, then there are actually only eight time slots during which the synchronisation channel can be received. In some HSCSD classes the time slots allocated to the traffic channel can not be used to receive neighbour base station information, because several extra time slots are allocated to the mobile station. In this case the timing of the received neighbour base station channels can become critical regarding the available time. The time slot of the received channel occurs either at the very beginning of the Idle period or at its end, whereby the frequency synthesiser has not enough time to perform the required frequency hops.
The FIG. 4 shows how a mobile station receives neighbour cell transmissions during the Idle period. In this example the synchronisation channel 43 and the control channel 44 occurs within this Idle period window there. The shown situation is most unfavourable, because only the reception of the control channel 44 is successful as there is sufficiently time on both sides of it for the frequency synthesiser to make the frequency hop. Thus the search for all channels of the neighbour cell base stations and the decoding of the information contained in them requires a lot of time in the mobile station. In the cases shown in FIGS. 3 and 4 a successful monitoring in a mobile station using the HSCSD protocol requires either the use of a very fast frequency synthesiser or one extra synthesiser only for the neighbour cell monitoring. Corresponding situations requiring a rapid frequency hop occur also in other HSCSD classes.
In the above described situations the mobile station does not have time to perform a perfect neighbour cell monitoring. In order to solve this problem the mobile station must be equipped with either a faster frequency synthesiser or with a second frequency synthesiser intended for monitoring purposes. However, the manufacturing of a very fast frequency synthesiser requires special components, and therefore the use of such a frequency synthesiser in conventional mobile stations would cause substantial extra costs. The addition of a second frequency synthesiser would also cause substantial extra costs. Secondly, the use of a fast frequency synthesiser or of two frequency synthesisers would increase the power required by the mobile station, which shortens the operational time of a mobile station equipped with a battery.